[GIS] Creating three polygons from two overlapping polygons in QGIS

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I have a set of polygons, several of which overlap. I want to process them so that (in the simplest case) two overlapping polygons become three non-overlapping polygons.

I can split up the polygons using the the Intersection tool, but this leaves me with the two original (and still overlapping) polygons, plus two new polygons corresponding to the overlap of each of the originals.

It feels like this should be a common enough task that there would be a built-in tool for it, but if so I can't find it.

Best Answer

Try using 'Polygon self-intersection' in the processing toolbox > SAGA-tools > Vector polygon tools. This function is implemented at least since QGis 2.14.15. Before:

It will do exactly what you want. All overlapping areas will be removed and replaced by a new geometry. The formerly overlapping parts will recieve concatenated field values. After:

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[GIS] way to split polygons by several lines in QGIS

The Digitizing Tools plugin will do the trick. One of the tools in that plugin is called "Split Selected Features With Selected Lines from Another Layer" enter image description here

[GIS] Merging overlapping polygons with original polygons in QGIS

The SAGA Polygon self-intersection would have created new ID field to store the combination of overlapped polygons. Please keep it when you export it as Polygon C.

Example output of Polygon self-intersection

With a bit of editing, this can be used as id field to put together with Polygon A (or Polygon B).

For instance, if we want to use second number from ID, expression like:

substr( "ID", strpos("ID", '|')+1, 1)

will return below.

(Please modify it to fit with your data. If you want to use the first one, it will be substr( "ID", strpos("ID", '|')-1, 1). )

After merging them, Collect geometries (QGIS 3.0) or Singleparts to multipart using this field (overlap in the above example) as Unique ID will do the job.

Best Answer

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